Method of annealing cold-rolling low-carbon steel sheets



Aug. 15, 1967 MINEO sHlMlzu ET AL 3,336,166

METHOD OF ANNEALING COLD-ROLLING LOW-CARBON STEEL SHEETS INVENTORS MINEO SHIMIZU KAMEO MATSUKURA NOBUYUKI TAKAHASH! YASUO SHlNAGAVVA BY Mwww M ATTORNEYS MiNEo si-iiMizU ET AL 3,336,166 METHOD OF ANNEALING COLD-ROLLING Aug. l5, 1967 L0w-cARBoN STEEL SHEETS 8 Sheets-Sheet 2;

Filed Aug. 2 4, 1964 in C/ hi? FiG. 2 B

O lOO ZOO BOO Heating .velocity (D) 50i Al 0.03-0.04 /o Heating velocity OO .a @n

Il C/A AA MIC/ md A we SS O mSm LES @Nm S9@ FIGZD INVENTORS MINEO SHIMIZU KAMEO MATSUKURA NOBUYUKi TAKAHASHI YASUO SHINAGAWA BY j ATTORNEYS Aug. l5, 1967 ConlcaI Cup value In mm l ComcaI CupvaIue In mm OJ MINEO sHlMl'zU ET AL METHOD OF ANNEALING COLD-ROLLING LOW-CARBON STEEL 'SHEETS 'Filed Aug. 24, 1964 SOI AI O. O3 /o Heaung veIocity In C/hr.

FIG. 5A

F IGG Conlcal Cup value in mm w pw C) O 8 Sheets-Sheet 5 SOI AI 0.03-0.04 0/0 0- SOI AIOOBZIIH) SOI AIOOBSIIY) VIGB O IOO 20D BOO Heating velooty m C/hr.

FIGS@ INVENTOR MINEO SI-IIMIZU KAMEO I/IATSUKURA NOBUYUKI 'TAKAI-IASI-II YASUO SHINAGAWA ATTORNEYS Augl5, 1967 MlNEo sl-uMlzu ET Al. 3,335,166

, METHOD OF ANNEALNG COLD-ROLLING LOW-CARBON STEEL SHEETS Filed Aug. 24, 1964 8 Sheets-Sheet 6 Heated at each heatln velocit from o the room temp. t0 7lgC and 3annealed f0l^ 4 hrs.

l-leacecll at each hea ln vel 't f 300 t0 550C and ac 0C Irrxl/n rger gergplso. ranges and annealed 7IO "C for S S0! Al 0.0200) 0.02901? H 0.03803) ClD Erichsen value in mm Conlcal cup valueln mm l l-leatln veloclty Heatln veloclt Heatln velocit ln" hr. l hwg/hf. y 'Wg/hn 'y FIG@ INVENTORS MINE-:O SHIMIZU KAMEO MATSUKURA NOBUYUK TAKAHASHI YASUO SHINAGAWA BY 67M ATTORNEYS 8 Sheets-Sheet 7 INVENTORS MINEO SHIMlZU KAM EO MATSUKURA NOBUYUKI TAKAHASH YASUO SHINAGAWA ATTORNEYS vMINE() SHIMIZU ETAL METHOD OF ANNEALING COLD-ROLLING LOW-CARBON STEEL SHEETS 50| Al 0.038%) (1y) BOO 500 Temp. In "C at the fmlsh of heatmg at 40C' hr.

FIG 7B Aug. 15, 1967 Filed Auvg, 24, 1964 O 1GO United States Patent `C) 3,336,166 METHOD OF ANNEALING COLD-ROLLING LOW-CARBON STEEL SHEETS Mineo Shimizu, Kameo Matsukura, Nobuyuki Takahashi,

and Yasuo Shinagawa, all of Kitakyushu, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan, a corporation of Japan Filed Aug. 24, 1964, Ser. No. 391,468 Claims priority, application Japan, Aug. 26, 1963, 38/ 45,598 11 Claims. (Cl. 14S-12.3)

` This invention relates generally to methods of annealing cold-rolled low-carbon steel sheets and more particularly to methods of annealing cold-rolled aluminumkilled low-carbon steel sheets to give them the highest press-formability and drawability.

This invention relates to methods of annealing coldrolled aluminum-killed low-carbon steel sheets containing 0.01 to 0.08% by weight acid-soluble aluminum by heating them in the range of 300 to 600 C. at a specific heating velocity in response to the content of acid-soluble aluminum in the steel by utilizing the recrystallizing performance characteristic of said steel sheets and annealing them at a temperature above 600 C. lbut below the AC3 point to give them the highest press-formability and drawability that can be expected of them.

It is a known fact that generally a metal will have such discontinuous physical changes as (l) recovery, (2) nucleation, (3) grain boundary movement and (4) crystal grain growth during its recrystallizing annealing. Whereas the recrystallization of cold-rolled rimmed low-carbon steel sheets will be completed quickly, that of cold-rolled aluminum-killed low-carbon steeel sheets will gradually proceed over a wide temperature range.

Generally, press-formability is largely divided into drawability and stretchability. For drawability, it is considered to be a requirement that steel sheets should have a plastic anisotropy. A plastic strain ratio is used to judge such plastic anisotropy. For stretchability, the requirements are considered to be that crystal grains should be larger (the yield ratio should be smaller) in case the material is the same and that the rate of elongation should be high. An Erichsen value is used to judge such elongation. A conical cup test or Fukui test are methods of testing mainly drawability and in addition stretchability. In most actual cases of press-forming, drawability and stretchability are both necessary. Therefore, in order to improve press-formability, both drawability and stretchability must be improved. However, these properties have physical signicances quite different from each other. It can not be always expected to simultaneously improve both properties, because, in the process of recrystallization, drawability is at least partly a function of the preferred crystal orientation but stretchability is at least partly a function of the property of the internal energy due to the variation of the strain state and the relation between the preferred crystal orientation and the property of the internal energy (for example, the crystal grain size) has not yet been well clarified.

However, it is already recognized that the recrystallizing performance of cold-rolled aluminum-killed lowcarbon steel sheets is different from that of cold-rolled rimmed low-carbon steel sheets and that the plastic anisotropy of cold-rolled aluminum-killed low-carbon steel sheets is stronger than that of cold-rolled rimmed low-carbon steel sheets. It is therefore evident that, in each of the above mentioned four physical changes (l) to (4) in the process of recrystallization, the recrystallizing performance will vary due to the amount of the solid-soluble element, the state of the deposit and the thermal condition. v

ice

Therefore, if conditions favourable to both drawability and stretchability are present in the recrystallizing process, both properties should be able to be improved. In practice it is very ditiicult to normally obtain steel sheets containing a narrow range (for example, 0.03 to 0.05%) of acid-soluble aluminum and adapted to a fixed annealing cycle. Therefore, in order to give the highest pressformability to each coil of a batch of aluminum-killed steel with a fluctuating content of acid-soluble aluminum, it is necessary to study the effect of thermal conditions on the drawability and stretchability in a wide range of contents of acid-soluble aluminum.

As a result of making experiments of the effect of thermal conditions on an ordinary aluminum-killed steel having an acid-soluble aluminum content in the range of 0.01 to 0.08% from the above mentioned viewpoint, we have discovered that, in recrystallizing, softening and annealing aluminum-killed low-carbon steel sheets above 600 C. but below the AC3 point, if a heating velocity in the range of 300 to 600 C. is specified in response to the content of acid-soluble aluminum, the highest pressformability will be obtained.

An object of the present invention is to provide an annealing method whereby the highest press-formability can be given to ordinary cold-rolled aluminum-killed lowcarbon steel sheets.

Another object of the present invention is to provide an annealing method whereby the highest press-formability can be given to cold-rolled aluminum-killed low-carbon steel sheets having a chemical composition of less than 0.1% carbon, 0.2 to 0.5% manganese and 0.01 to 0.08% acid-soluble aluminum which are generally used as deep-drawing steel sheets in which method the heating velocity is chosen in response to the content of acidsoluble aluminum.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawing.

The accompanying drawings illustrate various examples for explaining the formation of the present invention.

FIGS. lA-lD are graphs showing the influence of the heating velocity on the plastic strain ratio in steel sheets having various acid-soluble aluminum contents.

FIGS. 2A-2D, 3A-3D, 4A-4D and 5A-5D are graphs showing the influences of the heating velocity on the grain size, yield ratio, Erichsen value and conical cup value in steel sheets having various acid-soluble aluminum contents.

FIGURE 6 is a graph showing the comparison of the conical cup value and Erichsen value of steel sheets in which the heating velocity in the range from room temperature to 710 C. was controlled (marked with black circles) with the corresponding values for steel sheets in which the heating velocity was controlled only in the range of 300 to 550 C. but was 250 C./hr. in other temperature ranges (marked with white circles) as they were heated from the room temperature to 710 C., and in which the soaking time was 4 hours.

FIGS. 7A and 7B are graphs showing the inuence on the conical cup value and Erichsen value of the heating temperature range of heating at 40 C./hr. in annealing steel sheets having an acid-soluble aluminum content of 0.038% at 710 C. for 4 hours and in which in other temperature ranges, the steel sheets were heated at 250 C./hr.

FIGURE 8 is a graph showing the influence on the grain size, Erichsen value and conical cup value of the annealing temperature and time when steel sheets having acid-soluble aluminum contents of 0.029 and 0.038% were heated at 40 C./hr. in the temperature range of 3 400 to 550 C. and at 250 C./hr. in other temperature ranges.

The requirements of the method requirements of the present invention shall now be concretely explained with reference to the accompanying drawings. The present invention is to be applied to steel sheets produced by hotrolling and then cold-rolling an aluminum-killed low-carbon steel of a chemical composition of less than 0.10% carbon, 0.20 to 0.50% manganese and 0.01 to 0.08% acidsoluble aluminum, the rest being mostly iron and unavoidable impurities, which steel sheets are generally -used for deep drawing, and further is to be applied generally to unannealed coils of strips of cold-rolled aluminum-killed low-carbon steel sheets of the above mentioned composition reduced to the required thickness by pickling hotrolled coils of a xed thickness and then cold-rolling them above the ordinary cold-reduction rate of 40%. However, in order to better attain the object of the present invention by the annealing method of the present invention, it is desirable to make the cold-reduction rate 55 to 80%.

In the above mentioned chemical composition of the material to which the present invention is applied, if carbon is present in an amount more than 0.10%, the cementite will increase and the object of the present invention will not be attained. Further, if manganese is present in an amount less than 0.20%, hot-rolling brittleness will be produced and, if it is present in an amount more than 0.50%, hardening will occur.

Table 1 shows the chemical compositions, manufact-uring processes and histories of various examples of materials for explaining the present invention.

Next, as regards crystal grains, as shown by curves (I) and (Il) in FIG. 2A, first foi acid-soluble aluminum in an amount of 0.01 to 0.03%, the lower the heating velocity, the larger the crystal grains but, in this range of the amount of acid-soluble aluminum, when the heating velocity was higher than 20 C./hr., the crystal grains became smaller and the stretchability was not high. When the amount of acid-soluble aluminum was 0.002% as in (0) in FIG. 2A and was less than 0.01%, no substantial variation of crystal grains due to the heating velocity was seen. When the amount of acid-soluble aluminum was 0.03 to 0.04% as in FIG. 2B, at a heating velocity of 40 C./hr., the crystal grains became the largest. Further, when the amount of acid-soluble aluminum was 0.04 to 0.05% as in FIG. 2C, at a heating velocity of 150 C./hr. and when the amount of acid-soluble aluminum was more than 0.05% as in FIG. 2D, at a heating velocity of 250 C./hr., the crystal grains became the largest. The reason for this is also not clear but is considered to be due to the influence of obstacles to the grain boundary movement. The relation of the heating velocity to the yield ratio and Erichsen value correlated to some extent with the crystal grains is seen to have the same tendency as was seen in the correlation between the amount of acidsoluble aluminum and the heating velocity as regards the crystal grain, as is shown in FIGS. 3A-3D and 4A-4D.

From the above facts and the relation between the conical cup value and heating velocity as shown in FIGS. 5A-5D, it is found that, for the improvement of pressformability, the heating velocity must be specied in response to the amount of acid-soluble aluminum. That is TABLE l.-CHE1\IICAL COMPOSITIONS AND I-IISTORIES Chemical compositions in percent by weight Hot-rolling Hot-rolling Cold-reduc- Sample finishing ceiling tion rate in temperature temperature percent C Si Mn P S l Cu ,Sol Al in C. in C.

. 0. 32 0. 010 0. 017 0. 006 0. 002 860 600 G5 0.31 0.010 0.018 0. 065 0. 020 850 540 (i5 0. 33 0. 013 0. 019 0. 077 0. 029 860 540 (i5 0. 33 0. 011 0. 017 0. 054 0. 032 845 550 (i5 0.31 0. 009 0. 018 0. 059 0. 038 845 540 65 0.33 0. 013 0. 016 0. 055 0. 043 850 555 65 0.36 0. 014 0. 016 0. 052 0. 047 8G() 550 G5 0. 32 0. 011 0. 017 0. 056 0. 054 850 550 65 0. 011 0. 32 0. 010 0. 015 0. 050 0. OGG 848 550 65 0.013 0.32 0.011 0. 010 0. 054 0. 072 850 450 65 The sample No. 0 in Table 1 is rimmed steel in which the Content of acid-soluble aluminum is less than the lower limit of 0.01% in the present invention and therefore to which the annealing method of the present invention can not be effectively applied and which is shown for comparison. The samples Nos. I to IX are materials to which the present invention can be applied.

Now, these cold-rolled coils were bright-annealed at a temperature above 600 C. but below the AC3 transformation point for a proper time in an inert gas atmosphere in a proper annealing apparatus. In these examples, the material was annealed `at an annealing temperature of 710 C. for a soaking time of 4 hours in an inert (HNX) gas atmosphere of hydrogen and nitrogen in a box-annealing furnace and was cooled in the furnace. The annealing furnace is not limited to the box-annealing type but may be of any other type. The cooling velocity after the annealing has nothing to do with the substance of the present invention.

As a result, as regards plastic anisotropy, it was recognized that, as shown in FIGS. 1A-1D, the lower the heating velocity, the higher the plastic anisotropy and plastic strain ratio irrespective of the amount of acid-soluble aluminum.

The reason for this is not clear but is considered to be that the development of the crystal preferred orientation favourable to plastic anisotrophy was accelerated by reducing the heating velocity in the recrystallizing process.

to say, if the heating velocity for a steel sheet of an acid-soluble aluminum content having 0.01 to 0.03% is made less than 20 C./hr., there will be obtained a steel sheet high in both drawability and stretchability. When a steel sheet having an acid-soluble aluminum content of 0.03 to 0.04% is heated at 20 C. to 60 C./hr. or specfically near 40 C./hr., the stretchability will be the highest but the drawability wil not be the highest. However, if it is heated at 20 to 60 C./hr. or specifically near 40 C./hr., the conical cup value will be the highest. Therefore, in case press-formability including both properties of drawability and stretchability is taken into consideration, a heating velocity in the range of 20 t0 60 C./hr. or speciicaly near 40 C./hr. will be the best. By the same consideration, when a steel sheet of having acid-soluble aluminum content of 0.04 to 0.05 is heated at a heating velocity in the range of 60 to 200 C./hr., the pressformability will be improved. A heating velocity specifically from to 150 C./hr. is the best.

When a steel sheet having an acid-soluble aluminum content of more than 0.05 is heated at a velocity of 200 to 300 C./ hr., a steel sheet high in stretchability will be obtained. At 250 C./ hr., the stretchability will be the highest. On the other hand, with this steel sheet, the relation of the heating velocity with the plastic strain ratio and yield ratio will have a completely reverse tendency and, therefore the contribution from the two properties of drawability and stretchability will be canceled and the difference due to the heating velocity will not appear to be very great. Therefore, when the press-formability is taken into consideration, heating at 200 to 300 C./hr. or specifically near 250 C./hr. will be the best generally speaking. Further, an example of a conventional heating velocity in recrystallizing softening annealing above 600 C. but below the Ac3 point as mentioned above is about 710 C./hr. When the acid-soluble aluminum content of the steel sheet in the present invention is less than 0.01%, as understood from the above described examples, no significant dilference due to the heating velocity will appear in the properties of the steel sheet and a non-aging property will not be present. The addition of more than 0.08% acid-soluble aluminum will have no specific merit as far as non-aging and drawability are concerned but will have the disadvantage that the crystal grains will become fine. Therefore, the amount of acid-soluble aluminum is limited to from 0.01 to 0.08%.

Further, even if the temperature range for heating at a specific heating velocity is -from 300 to 600 C. and heating is carried out at any heating velocity in other temperature ranges as shown in FIGURES 6 and 7, there will be no interference at all with the object of the present invention. This fact makes possible a reduction of theannealing time and requires no extraordinarily long annealing cycle for a specic temperature raising velocity.

As understood also from FIGURE 8, if the heating velocity is proper, the annealing temperature and time will have little inliuence on the material of the steel sheet However, if the annealing temperature is below 600 C., time will be required for recrystallization and the improvement of mechanical properties will not be suiiicient. Further, above the AC3 point, austenite will be formed and the plastic anisotropy will be lost.

The recrystallized grain of steel sheets annealed under the above mentioned proper conditions have an axis ratio of 2.3 to 3.2 and have a so-called pancake or elongated shape grain.

While the present invention has been specilically described and illustrated herein with reference to the preferred embodiments of the invention, it is to be understood that the present invention may be otherwise practiced than as specifically described and illustrated within the scope and spirit of the appended claims.

What We claim is:

1. In a method of annealing cold-rolled low-carbon steel sheets to obtain optimum drawability and stetchability wherein a cold-rolled -aluminum-killed 10W-carbon steel sheet containing less than 0.10% carbon, 0.20 to 0.50% manganese, and 0.01 to 0.08% acid soluble aluminum, the rest being iron and unavoidable impurities is annealed at temperatures in the range of about 300 C. to about 600 C. and then recrystallized at a temperature above about 600 C. and below the AC3 point, the improvement comprising selecting a heating rate of from about 20300 C. per hour in a range of about 300-600" C. in respect to the acid soluble aluminum content.

2. The improvement as claimed in claim 1, in which when the aluminum content is 0.01-0.03% and the heating rate is about 20 C. per hour.

3. The improvement as claimed in claim 1 in which when the aluminum content is 0.03 to 0.04% and the heating rate is from 20-60 C. per hour.

4. The improvement as claimed in claim 1, in which when the aluminum content is 0.040.05% and the heating rate s from -200" C. per hour.

5. The improvement as claimed in claim 1 in which when the aluminum content is 0.05-0.08% and the heating rate is 4from 200-300 C. per hour.

6. The improvement as claimed in claim 1 in which the acid-soluble aluminum content is from 0.03% to 0.04% and the heating velocity is about 40 C. per hour.

7. The improvement -as claimed in claim 1 in which the acid-soluble aluminum content is from 0.04% to 0.05% and the heating velocity is about C. per hour.

8. The improvement as claimed in claim 1 in which the acid-soluble aluminum content is about 0.05% and the heating velocity is about 250 C. per hour.

9. The improvement as claimed in claim 1 in which the cold-rolled sheet steel is cold rolled at a reduction rate of more than 40% prior to the annealing.

10. The improvement as claimed in claim 9 in which the `cold-rolling reduction rate is from 55 to 80%.

11. The improvement as claimed in claim 1 in which the heating velocity is about 40 C. per hour in the range of from 300 C. to 550 C., and the recrystallizing annealing temperature is 710 C.

References Cited UNITED STATES PATENTS 5/1952 Darmara 148-12 5/1961 Leslie et al 148-36 X OTHER REFERENCES DAVID L. RECK, Primary Examiner.

CHARLES LOVELL, Examiner. 

1. IN A METHOD OF ANNEALING COLD-ROLLED LOW-CARBON STEEL SHETS TO OBTAIN OPTIMUM DRAWABILITY AND STETCHABIITY WHEREIN A COLD-ROLLED ALUMINUM-KILLED LOW-CARBON STEEL SHEET CONTAINING LESS THAN 0.10% CARBON, 0.20 TO 0.50% MANGANESE, AND 0.01 TO 0.08% ACID SOLUBLE ALUMINUM, THE REST BEING IRON AND UNAVOIDABLE IMPURITIES IS ANNEALD AT TEMPERATURES IN THE RANGE OF ABOUT 300 C. TO ABOUT 600*C. AND THEN RECRYSTALLIZED AT A TEMPERATURE ABOVE ABOUT 600*C. AND BELOW THE AC3 POINT, THE IM- 